WO2009040842A1

WO2009040842A1 - Apparatus for simulating a non-linear load, to be connected to an electric distribution network

Info

Publication number: WO2009040842A1
Application number: PCT/IT2008/000427
Authority: WO
Inventors: Mario La Rosa; Pier Angelo Montenegro; Prospero Capogrosso; Angelo Euplio D'ambrosio
Original assignee: Energhes S.R.L.; Mesar S.R.L.
Priority date: 2007-09-24
Filing date: 2008-06-24
Publication date: 2009-04-02
Also published as: ITRM20070494A1

Abstract

The present invention concerns an apparatus (1 ) for simulating a non-linear load to be connected to an electric distribution network (2), said apparatus (1 ) being provided with a neutral terminal (3) and with one or more terminals (4\ 4' and 4'') for connection with said electric distribution network (2), each one of said connection terminals corresponding to a phase of the same network, and comprising a control logic unit (16) connected with detection means (18) for one or more operation parameters of said apparatus (1), characterized in that it further comprises an inlet inductive unit (5) connected with said one or more terminals (4', 4' and 4'', 3); one or more inverter block (7), in number corresponding to the phases of said electric distribution network (2), connected with said inlet inductive unit (5), each inverter block (7) comprising first switching means (8), the control terminal of which is connected with said control logic unit, and having in parallel a diode (9); capacitive means (10), connected in parallel with said one or more inverter blocks (7), said inverter blocks (7), cooperating with said inlet inductive unit (5), being suitable to adjust absorption of reactive power and distorting power of said apparatus (1) by variation of potential at the ends of the capacitive means (10); and one or more chopper circuits (11), comprising dissipation means (12) and second switching means (13), the control terminal of which is connected with said logic control unit (16).

Description

APPARATUS FOR SIMULATING A NON-LINEAR LOAD, TO BE CONNECTED TO AN ELECTRIC DISTRIBUTION

NETWORK

The present invention relates to an apparatus for simulating a non-linear load to be connected to an electric distribution network.

More specifically, the invention concerns an apparatus able simulating an electric load adjusting absorption or current generation according to a pre-set curve, simulating effects of connection of said electric load to a known complex network of an unknown power distribution network, so as to eventually permitting designing of a suitable adaptation network in order to obtain an efficient power transfer.

As it is well known, in the last years, problems caused by "Power Quality" interferences in the field of designing power electric network have always more become critical, also in view of the growing diffusion of electronic components.

Usually, typical problems in delivering electric power are, for example: voltage fluctuations; micro interruptions; current and voltage harmonics; high peak values (typically about 2.5 - 3); low power factors (typically about 0.7); high current in ground wires due to the unbalance and composition of third order harmonics; electromagnetic noise irradiated; distortion of the supply voltage; low MTBF (Mean Time Between Failures); low efficiency and need of over sizing the electric apparatus; over sizing of electric commutation devices; over sizing of power cables; need of overcurrent protection for ground wire; need of special shields and filters for power and signal network; malfunction of current switches.

The above phenomenon can even meaningfully influence the proper working of apparatus components until compromising standard manufacturing process with remarkable economic consequences in the worst cases.

For example, it often occurs in connection of power electric devices with three phase industrial networks. In fact, it is well known that 50/60 Hz alternate supply network does not have a perfectly sinusoidal voltage wave shape, i.e. a power spectrum with a single harmonic at 50/60 Hz. Due to the above complexity of the electric network, electric network supply wave shape has a current spectrum with a plurality of spurious harmonics, often with a not negligible amplitude. Therefore, due to the above complexity of the user networks, effect due to the modification of the same by connection with a non-linear load often cannot be foreseen and, as already said, sometimes have very serious consequences on systems or on the connected load.

It is well evident that the above procedure is economically onerous and does not ensure acceptable safety standards.

At present, network designers tend to face the above problems by different methods, such as the use of traditional LC passive filters, special transformers or active techniques. LC filters can be conformed to a low power device, but, as it is well known, they are sensible to the load variations and to voltage distortions. The same problem occurs with special transformers.

Italian patent application n° TO2004A000020 faces the above problems. Said patent application concerns an apparatus simulating complex electric loads permitting absorbing a current from an electric supply network according to a wished curve by control means for a resistive load and an adjustable active filter.

However, it has been observed that the above apparatus has technical and precision limitations when simulating the different possible wave shape that can be absorbed (or generated) by the supply network (and thus in realizing spectra absorbing the same).

In view of the above, it is object of the present invention that of suggesting an apparatus for testing connection of a not-linear load to supply network, permitting testing properties of said electric distribution network and of its components, so as to detect performances both in standard and anomalous conditions, and to permit designing possible modifications and integrations necessary to interface said supply electric network with said known absorption or generation load.

Another object of the invention is that of permitting testing system and load components, either for designing them or for verifying conformity of the latter.

A further object of the present invention is that of permitting a high precision in simulating current absorption or generation according to a frequency spectrum and running that can be pre-set with reference to the real absorption or generation load to be connected. It is therefore specific object of the present invention an apparatus for simulating a non-linear load to be connected to an electric distribution network, said apparatus being provided with a neutral terminal and with one or more terminals for connection with said electric distribution network, each one of said connection terminals corresponding to a phase of the same network, and comprising a control logic unit connected with detection means for one or more operation parameters of said apparatus, characterized in that it further comprises an inlet inductive unit connected with said one or more terminals; one or more inverter block, in number corresponding to the phases of said electric distribution network, connected with said inlet inductive unit, each inverter block comprising first switching means, the control terminal of which is connected with said control logic unit, and having in parallel a diode; capacitive means, connected in parallel with said one or more inverter blocks, said inverter blocks, cooperating with said inlet inductive unit, being suitable to adjust absorption of reactive power and distorting power of said apparatus by variation of potential at the ends of the capacitive means; and one or more chopper circuits, comprising dissipation means and second switching means, the control terminal of which is connected with said logic control unit, suitable to adjust current intensity running through said dissipation means, said at least one chopper circuit being connected in parallel with said one or more inverter blocks and suitable to adjust active power dissipation of said apparatus, so that said control logic unit pilots said first and second switching means absorbing or generating current from said electric distribution network according to a pre-set frequency absorption spectrum.

Always according to the invention, said inlet inductive unit can comprise, for each phase of said electric distribution network, an inductance connected with each terminal, suitable to permit, by variation of load, and thus of tension at the ends of said capacitive means, adjustment of absorbed or generated current by each phase of said electric distribution network. Still according to the invention, said inlet inductive unit can be apt to filtering high frequency noise generated by said apparatus and realizing, for each phase, by said one or more inverter blocks and said one or more chopper circuits, a low-pass frequency filtering.

Furthermore, according to the invention, said low-pass frequency filtering can have such a cut frequency to permit passage of harmonics up to the 40^th of the nominal basic frequency of said electric distribution network. Preferably, according to the invention, said cut frequency can be of 2 KHz.

Advantageously according to the invention, said detection means can comprise means for measuring current connected with said one or more terminals, apt to detect current entering in said electric distribution network, means for measuring voltage of said one or more terminals with respect to a reference potential, and means for measuring voltage at the ends of said capacitive means.

Always according to the invention, said second switching means can be connected in series with said dissipation means.

Still according to the invention, said second switching means can be connected in parallel with said dissipation means.

Further according to the invention, said first and second switching means can be piloted by PWM (Pulse Width Modulation). Preferably, according to the invention, said first and second switching means can be solid state switches, particularly IGBT (Insulated Gate Bipolar Transistor).

Always according to the invention, said first switching means and said diodes of each inverter block can be connected according to an NPC (Neutral Point Clamp) mode.

Still according to the invention, said apparatus can comprise an interface unit connected with said control logic unit, apt to permit setting of current to be absorbed or generated to said electric distribution network and to permit visualization in real time of current and voltage entering within the apparatus.

Preferably, according to the invention, said apparatus can comprise an auxiliary feeder of the same apparatus.

Furthermore according to the invention, said control logic unit can comprise a digital signal processor (DSP). The present invention will be described for illustrative and not limitative purposes according to its preferred embodiments, with particular reference to the figures of the enclosed drawings, wherein: figures 1a and 1b show electric schemes of the operation principle of a non-linear load simulation apparatus to be connected to an electric distribution network according to the present invention; figure 2 shows a block diagram of a non-linear load simulation apparatus to be connected to an electric distribution network according to the present invention; figure 3 shows a first embodiment of apparatus according to the present invention; figure 4 shows a second embodiment of apparatus according to the present invention; figure 5 shows a third embodiment of apparatus according to the present invention; figure 6 shows a fourth embodiment of apparatus according to the present invention; figure 7 shows a fifth embodiment of apparatus according to the present invention; and figure 8 shows a sixth embodiment of apparatus according to the present invention.

Basic idea on which it is based the apparatus for simulation of a non-linear load according to the present invention is that of creating a stable load absorbing:

• a set current; • an active power;

• a reactive power; and

• a distorting power.

To this end, the system that can better characterize a load as defined in the above is an ideal current generator. Starting from the circuit equivalence shown in figure 1a, by which a current generator i/(t) with a capacity C connected in parallel, is equivalent, if suitably dimensioned, to a voltage generator v(t) to which an inductance L is connected in series, it is possible obtaining a stable load as defined in the above, making reference to the principle diagram shown in figure 1b, wherein V_gen(t) is voltage supplied by the load network, and Vβus(t) is voltage at the ends of the preloaded condenser C, represented as an ideal voltage generator. i*(t) is, with the suitable sign, current that it is wished generating or absorbing, so as to simulate the load behavior.

Circuit 1b relationships are:

.i^v _«Λ(0 -v_flrø(0]9r

from which it is obtained that controlling V_Bus(t), i.e. condenser C load, it is possible obtaining wished current i^*(t) and simulating absorption or generation of the wished load.

In other words, it is controlled Vβus(t) so as to control voltage variation V_gen(t) - V_Bus(t) and thus its integral equal to current i*(t). Inductance L thus permits transforming voltage generator into a current generator. For absorbing active power, it is sufficient connecting in parallel to condenser C a resistive load and a chopper circuit controlling discharge of energy stored within condenser C on the same, taking into consideration time constant generated.

Preferred circuit structure to realize the above circuit structure is based on a bridge of active switches (preferably IGBT switches - Insulated Gate Bipolar Transistor) controlled, for example, by PWM (Pulse Width Modulation) modulation, current controlled, closed on a condenser and a resistance. The latter is apt to absorb the first harmonic component phased with the supply voltage. On the basis of the above, making reference to figure 2, it is possible observing a block diagram of apparatus 1 for simulation of a nonlinear load to be connected to an electric distribution network 2, implementing the above principle schemes.

Said apparatus 1 mainly comprises an inlet inductive unit 5, connected with inversion means or an inverter, and a chopper circuit.

As it can be noted, said inlet inductive unit 5 is connected to a three phase electric distribution network 2 by a neutral terminal 3 and three terminals 4', 4", 4'".

Inlet inductive unit 5 is comprised of inductances 6, each one connected to each terminal 4', 4", 4"' of each phase and on neutral terminal 3. Said inductances 6 act as inductance K described in figure 1b. Said inlet inductive unit 5 also has the function of filtering noise generated by power stage of the same apparatus 1. In fact, said inlet inductive unit 5 permits realizing a low-pass frequency filtering at a cut frequency to permit passage of a harmonic until the 40^th of the basic one. In case of common mono-three phase supply network, base frequency is equal to 50/60 Hz. It implies that said inlet inductive unit 5 cooperates realizing a cut frequency higher than 2 KHz.

One or more inverter blocks 7, or inverter bridges, are connected in cascade to the inlet inductive unit 5, for each phase of said electric distribution network. Each one of said inverter blocks 7 is comprised of first active switches 8 and diodes 9, connected to said active switches 8 according to an NPC (Neutral Point Clamp) mode. This configuration, with respect to the classic half bridge configuration, has been suggested by the searching of a higher efficiency. In fact, NPC configuration permits obtaining higher working powers and frequencies with respect to the half bridge configuration since active switches 8 are subjected to a lower stress with respect to commutations. However, present configuration with respect to the standard one requires a higher number of active switches 8.

Capacity 10 and a chopper circuit 11 , comprising dissipation resistances 12 and second active switches 13 are in parallel to said inverter blocks 7.

Apparatus 1 can be indifferently supplied according to two alternative modes: by an auxiliary feeder 14, to compensate the apparatus

1 self-consumption dissipations, so that they do not influence reproduction or simulation of absorption or generation by apparatus 1 , thus permitting reproduction of a purely reactive current; or, by one of the phases of the electric distribution network 2, to which said apparatus 1 is connected. Choice of the preferred supply mode occurs by a switch 15.

Apparatus 1 according to the present invention also includes a control logic unit 16, connected with:

- terminals 3, 4', 4", 4"', by suitable first detection means (not shown in figure 2) to detect potential and current through the same;

- at the ends of capacity 10 and dissipation resistances 12, to detect potential differences;

- control terminals of active switches 8 and 13, in order to control activation, preferably by PWM signals. Said control logic unit 16 is provided with a digital signal microprocessor (DSP) wherein measurements carried out on inlet magnitude converge, power handled by inverter blocks 7, and programmable load operation environmental conditions, and possible further information that can be input by user. Microprocessor processes data provided and generates control signals for active switches 8 and 13, and possibly interrupts operation of apparatus 1 , in case of malfunctioning. Apparatus 1 further comprises an interface unit 17 connected to and controlled by said control logic unit 16.

User interface 17 of the control logic unit in the present embodiment is a processor, preferably with a touch screen interface, able visualising on a screen (not shown in figure 2) run of current and voltage waveforms entering into apparatus 1 , while operating as programmable load, detected by control logic unit 16.

Said interface nit 17 permits to a user to set current to be absorbed or generated by the tested electric distribution network 2. Furthermore, it also permits real time visualization of run of inlet current and voltage to apparatus 1.

Operation of apparatus 1 according to the invention is as follows. As already said, apparatus 1 absorbs (or generates) from three- phase electric distribution network 2 a current with a basic frequency at 50/60 Hz and harmonics until 40^th. Characteristics of absorbed (or generated) current can be set by the user during the preparation step of the connection test by interface unit 17. In other words, control logic unit 16 is programmed so as to make the apparatus taking frequency behaviour of the transfer function or the real load that it is wished connecting, i.e. make it absorbing or generating a current according to a pre-set time function.

Control logic unit 16 processes data input by the user, evaluates its corresponding to the operation specification and carries out the required operation. At the same time, it is detected, in real time by said detection time, operation state of apparatus 1. Once preloaded, capacity 10 behaves as a constant voltage generator. Control logic unit 16 pilots said first active switches 6 and said second active switches 13 so as to absorb a current from electric distribution network 2 according to a wished wave shape (and thus according to the pre-set spectrum), just modifying potential at the ends of capacity 10, constantly monitored by control logic unit 16, by inverter blocks 7. In the same way, it is also adjusted current absorption (or generation), i.e. dissipation of active power on the dissipation resistances 2 for the first harmonic and storage/delivery of reactive energy for each one of the other selected harmonics (40^th in the present embodiment) by apparatus 1 , so as to reproduce the response of the transfer function of the real load that it is wished connecting to the supply network 2.

Thus, control logic unit 16 can be very precisely adjusted absorption (or generation) of current, reactive power and distorting power. Among the checked magnitude, we can mention voltage and absorbed (generated) current that are sent in real time to the interface unit 17 to be displayed. Instantaneous tension and inlet current values and current set by the user are processed to generate piloting signals of the first active switches 8 of inverter blocks 7.

Monitoring of charge of condenser downward inverter blocks 7 permits generating control signals of second active switches 13, so as to adjust dissipation of active power required during the test by supply by dissipation resistances 12. At the same way, piloting of first active switches 8 permits simulating absorption or cession of reactive power and distorting power, controlling load of capacity 10 and their discharge on the dissipation resistances 12.

Figure 3 shows a first embodiment of apparatus 1 according to the invention.

In this case, electric distribution network 2 is still a three-phase network, while six inverter blocks 7 are provides, three of which having dissipation resistances 12 at their ends, while the others have capacity 10 at their ends.

Active switches are IGBT, which are known as having a high commutation speed.

Furthermore, control logic unit 16 has: • a first connection channel or bus 16a connected with current detectors 18, connected with the inlet of the inductive unit 5 for detection of current; • a second connection channel or bus 16a', connected with voltage detectors with respect to a reference point, connected with the inlet of the inductive unit 5; • a third connection channel or bus 16a" connected with voltage detectors with respect to a reference point, connected to the ends of resistances 12 and capacity 10;

.• a fourth connection channel or bus 16b connected with control terminals or basis of transistors 8 and 13 for piloting the same.

Piloting transistors 8 and 13 by bus 16b (control bus) and processing current and voltage measures acquired by bus 16a, 16a', 16a", control logic unit 16 can reproduce current absorption or generation for the insulated spectrum by inlet inductive unit 5.

Figure 4 shows a further embodiment of the present invention, wherein:

• two separate active switches or transistors 19 have been added to each capacity 10, each one having a diode 19' connected in parallel to the emitter - collector junction;

• a diode 13' has been added in parallel to the emitter - collector junction of each transistor 13.

Figure 5 shows a third embodiment of an apparatus according to the invention. The latter, for each phase of the electric distribution network 2 has an inverter block 7 wherein, it is present in parallel capacity 10 and dissipation resistance 12.

Embodiment of figure 6 is substantially identical to the embodiment of figure 5, being different in that it is provided a series connection of a transistor 19 and of a relevant diode 19' with each capacity 10 of each inverter block 7.

Instead, figure 7 has, with respect to figure 6, in lieu of transistor 19 and of diode 19', transistor 20, and relevant diode 20', connected in parallel with capacity 10 of each inverter block 7.

Figure 8 shows the last embodiment of the present invention, wherein, at the ends of capacity 10 of each inverter block, there are connected:

• a transistor 19 and the relevant 19', connected in series;

• a transistor 20 and the relevant diode 20', connected in parallel. Coming back to basic diagram of figure 3, as already said, absorbed or generated current, and voltage applied to apparatus 1 , thus acting as programmable load, occurs at the inlet of the inlet inductive unit 5, permitting monitoring effect on the electric distribution network 2 (source) to be tested.

Once tests have been carried out on the apparatus 1 connection with the electric distribution network 2, by data collected and examined by the interface unit 17, operator can very safely connect real load to be connected or design a suitable adjustment network, in order to prevent unwished phenomenons.

On the basis of the above specification, it can be noted that basic feature of the present invention is that of suggesting a kind of circuit for absorbing programmable active, reactive and distorting power, permitting simulation of absorption (generation) of current with respect to a frequency spectrum of each complex non-linear load, so as to verify effect on the electric distribution network to which it is connected.

The present invention has been described for illustrative but not limitative purposes, according to its preferred embodiments, but it is to be understood that modifications and/or changes can be introduced by those skilled in the art without departing from the relevant scope as defined in the enclosed claims.

Claims

1. Apparatus (1) for simulating a non-linear load to be connected to an electric distribution network (2), said apparatus (1) being provided with a neutral terminal (3) and with one or more terminals (4', 4" and 4"') for connection with said electric distribution network (2), each one of said connection terminals corresponding to a phase of the same network, and comprising a control logic unit (16) connected with detection means (18) for one or more operation parameters of said apparatus (1), characterized in that it further comprises an inlet inductive unit (5) connected with said one or more terminals (4', 4" and 4"', 3); one or more inverter block (7), in number corresponding to the phases of said electric distribution network (2), connected with said inlet inductive unit (5), each inverter block (7) comprising first switching means (8), the control terminal of which is connected with said control logic unit, and having in parallel a diode (9); capacitive means (10), connected in parallel with said one or more inverter blocks (7), said inverter blocks (7), cooperating with said inlet inductive unit (5), being suitable to adjust absorption of reactive power and distorting power of said apparatus (1) by variation of potential at the ends of the capacitive means (10); and one or more chopper circuits (11), comprising dissipation means (12) and second switching means (13), the control terminal of which is connected with said logic control unit (16), suitable to adjust current intensity running through said dissipation means (12), said at least one chopper circuit (11) being connected in parallel with said one or more inverter blocks (7) and suitable to adjust active power dissipation of said apparatus (1), so that said control logic unit (16) pilots said first and second switching means (8, 13) absorbing or generating current from said electric distribution network (2) according to a pre-set frequency absorption spectrum.

2. Apparatus (1) according to claim 1 , characterized in that said inlet inductive unit (5) comprises, for each phase of said electric distribution network (2), an inductance (6) connected with each terminal (4', 4" and 4"', 3), suitable to permit, by variation of load, and thus of tension at the ends of said capacitive means (10), adjustment of absorbed or generated current by each phase of said electric distribution network (2).

3. Apparatus (1) according to one of the preceding claims, characterized in that said inlet inductive unit (5) is apt to filtering high frequency noise generated by said apparatus (1) and realizing, for each phase, by said one or more inverter blocks (7) and said one or more chopper circuits (11), a low-pass frequency filtering.

4. Apparatus (1) according to claim 3, characterized in that said low-pass frequency filtering can have such a cut frequency to permit passage of harmonics up to the 40^th of the nominal basic frequency of said electric distribution network (2).

5. Apparatus (1) according to claim 4, characterized in that said cut frequency is of 2 KHz.

6. Apparatus (1) according to one of the preceding claims, characterized in that said detection means comprise means for measuring current (18) connected with said one or more terminals (4' 4", 4"'), apt to detect current entering in said electric distribution network (2), means for measuring voltage of said one or more terminals (4' 4", 4"') with respect to a reference potential, and means for measuring voltage at the ends of said capacitive means (10).

7. Apparatus (1) according to one of the preceding claims, characterized in that said second switching means (13) are connected in series with said dissipation means (12).

8. Apparatus (1) according to one of the preceding claims 1 - 6, characterized in that said second switching means (13) are connected in parallel with said dissipation means.

9. Apparatus (1) according to one of the preceding claims, characterized in that said first and second switching means (8, 13)are piloted by PWM (Pulse Width Modulation).

10. Apparatus (1) according to one of the preceding claims, characterized in that said first and second switching means (8, 13) are solid state switches.

11. Apparatus (1) according to claim 10, characterized in that said first and second switching means (8, 13) are IGBT (Insulated Gate

Bipolar Transistor).

12. Apparatus (1) according to one of the preceding claims, characterized in that said first switching means (8) and said diodes (9) of each inverter block are connected according to an NPC (Neutral Point Clamp) mode..

13. Apparatus (1) according to one of the preceding claims, characterized in that it comprises an interface unit (17) connected with said control logic unit (16), apt to permit setting of current to be absorbed or generated to said electric distribution network and to permit visualization in real time of current and voltage entering within the apparatus.

14. Apparatus (1) according to one of the preceding claims, characterized in that it comprises an auxiliary feeder (14) of the same apparatus (1).

15. Apparatus (1) according to one of the preceding claims, characterized in that said control logic unit (16) comprises a digital signal processor (DSP).